1. Field of the Invention
The present invention is directed to a pressure-limiting valve for use in a gas system, such as for manual ventilation of a patient.
2. Description of the Prior Art
Manual ventilation of patients is very common in conjunction with anaesthesia administration. Physicians employ manual ventilation of the patient especially at the start and end of anaesthetization. The physician then uses a manually operated gas system, with a hand ventilator and an adjustable, pressure-limiting valve for controlling the pressure of the breathing gas in the manually operated gas system. The adjustable pressure-limiting valve is often referred to as an APL valve, the term that will be used henceforth in this description.
In principle, the APL valve includes a valve seat and a valve plate which, in the closed position, seals against the valve seat and which is lifted by positive pressure off the valve seat to release surplus gas. An adjustable valve spring exerts pressure on the valve plate with a force corresponding to the pressure set by the physician. Since the APL valve must open to release surplus gas at low positive pressures (about 3 cmH.sub.2 O above atmospheric pressure), the force on the valve plate must not be too large at the start of the setting range. In principle, the APL valve should not offer more resistance than is necessary to enable the hand ventilator just to be filled. In particular, it must not pose major resistance to patient expirations. One problem that can develop with APL valves is that they can begin oscillating, under certain flow and external conditions (with tubes, lines, other valves etc.), in such a way that an annoying noise is generated.